Haskell is based on the semantics, but not the syntax, of the language Miranda, which served to focus the efforts of the initial Haskell working group.[32] Haskell is used widely in academia[33][34] and industry.[35]

Haskell 1.0 to 1.4

The first version of Haskell ("Haskell 1.0") was defined in 1990.[1] The committee's efforts resulted in a series of language definitions (1.0, 1.1, 1.2, 1.3, 1.4).

Haskell 98

In late 1997, the series culminated in Haskell 98, intended to specify a stable, minimal, portable version of the language and an accompanying standard library for teaching, and as a base for future extensions. The committee expressly welcomed creating extensions and variants of Haskell 98 via adding and incorporating experimental features.[36]

In February 1999, the Haskell 98 language standard was originally published as The Haskell 98 Report.[36] In January 2003, a revised version was published as Haskell 98 Language and Libraries: The Revised Report.[27] The language continues to evolve rapidly, with the Glasgow Haskell Compiler (GHC) implementation representing the current de facto standard.[37]

Haskell 2010

In early 2006, the process of defining a successor to the Haskell 98 standard, informally named Haskell Prime, began.[38] This was intended to be an ongoing incremental process to revise the language definition, producing a new revision up to once per year. The first revision, named Haskell 2010, was announced in November 2009[2] and published in July 2010.

Haskell 2010 adds the foreign function interface (FFI) to Haskell, allowing for bindings to other programming languages, fixes some syntax issues (changes in the formal grammar), and bans so-called n-plus-k-patterns, that is, definitions of the form fact (n+1) = (n+1) * fact n are no longer allowed. It introduces the Language-Pragma-Syntax-Extension which allows for code designating a Haskell source as Haskell 2010 or requiring certain extensions to the Haskell language. The names of the extensions introduced in Haskell 2010 are DoAndIfThenElse, HierarchicalModules, EmptyDataDeclarations, FixityResolution, ForeignFunctionInterface, LineCommentSyntax, PatternGuards, RelaxedDependencyAnalysis, LanguagePragma and NoNPlusKPatterns.[2]

Code examples

Here is the factorial function in Haskell, defined in a few different ways:

-- Type annotation (optional)factorial::(Integrala)=>a->a-- Using recursionfactorialn|n<2=1factorialn=n*factorial(n-1)-- Using recursion, with guardsfactorialn|n<2=1|otherwise=n*factorial(n-1)-- Using recursion but written without pattern matchingfactorialn=ifn>0thenn*factorial(n-1)else1-- Using a listfactorialn=product[1..n]-- Using fold (implements product)factorialn=foldl(*)1[1..n]-- Point-free stylefactorial=foldr(*)1.enumFromTo1

The Int type refers to a machine-sized integer (used as a list subscript with the !! operator), while Integer is an arbitrary-precision integer. For example, using Integer, the factorial code above easily computes factorial 100000 as a huge number, of 456,574 digits, with no loss of precision.

This is an implementation of an algorithm similar to quick sort over lists, in which the first element is taken as the pivot:

quickSort::Orda=>[a]->[a]quickSort[]=[]-- The empty list is already sortedquickSort(x:xs)=quickSort[a|a<-xs,a<x]-- Sort the left part of the list++[x]++-- Insert pivot between two sorted partsquickSort[a|a<-xs,a>=x]-- Sort the right part of the list

The Utrecht Haskell Compiler (UHC) is a Haskell implementation from Utrecht University.[47] It supports almost all Haskell 98 features plus many experimental extensions. It is implemented using attribute grammars and is currently mainly used for research into generated type systems and language extensions.

Jhc is a Haskell compiler written by John Meacham emphasising speed and efficiency of generated programs, and exploring new program transformations.

Ajhc is a fork of Jhc.

LHC is a whole-program optimizing backend for GHC. It is based on Urban Boquist’s compiler intermediate language, GRIN.[48] Older versions of LHC were based on Jhc rather than GHC.

Implementations no longer being actively maintained include:

The Haskell User's Gofer System (Hugs) is a bytecode interpreter. It used to be one of the implementations used most widely, alongside the GHC compiler,[49] but has now been mostly replaced by GHCi. It also comes with a graphics library.

nhc98 is a bytecode compiler focusing on minimizing memory use.

The York Haskell Compiler (Yhc) was a fork of nhc98, with the goals of being simpler, more portable and efficient, and integrating support for Hat, the Haskell tracer. It also had a JavaScript backend, allowing users to run Haskell programs in web browsers.

HBC is an early implementation supporting Haskell 1.4. It was implemented by Lennart Augustsson in, and based on, Lazy ML. It has not been actively developed for some time.

Implementations not fully Haskell 98 compliant, and using a variant Haskell language, include:

Gofer was an educational dialect of Haskell, with a feature called constructor classes, developed by Mark Jones. It was supplanted by Hugs (see above).

Helium is a newer dialect of Haskell. The focus is on make learning easier via clearer error messages. It currently lacks full support for type classes, rendering it incompatible with many Haskell programs.

Applications

Darcs is a revision control system written in Haskell, with several innovative features, such as more precise control of the patches to be applied. Cabal is a tool for building and packaging Haskell libraries and programs.[50]Linspire GNU/Linux chose Haskell for system tools development.[51]Xmonad is a window manager for the X Window System, written fully in Haskell.[52] GHC is also often a testbed for advanced functional programming features and optimizations in other programming languages. Pandoc is a tool to convert one markup format into another.

Industry

Bluespec SystemVerilog (BSV) is a language for semiconductor design that is an extension of Haskell. Also, Bluespec, Inc.'s tools are implemented in Haskell.

Cryptol, a language and toolchain for developing and verifying cryptography algorithms, is implemented in Haskell.

The first formally verifiedmicrokernel,[55]seL4, used Haskell as a prototyping language for the OS developer.[55]:p.2 At the same time the Haskell code defined an executable specification with which to reason, for automatic translation by the theorem-proving tool.[55]:p.3 The Haskell code thus served as an intermediate prototype before final C refinement.[55]:p.3

Web

Criticism

Jan-Willem Maessen, in 2002, and Simon Peyton Jones, in 2003, discussed problems associated with lazy evaluation while also acknowledging the theoretical motives for it,[58][59] in addition to purely practical considerations such as improved performance.[60] They note that, in addition to adding some performance overhead, lazy evaluation makes it more difficult for programmers to reason about the performance of their code (particularly its space use).

Bastiaan Heeren, Daan Leijen, and Arjan van IJzendoorn in 2003 also observed some stumbling blocks for Haskell learners: "The subtle syntax and sophisticated type system of Haskell are a double edged sword – highly appreciated by experienced programmers but also a source of frustration among beginners, since the generality of Haskell often leads to cryptic error messages."[61] To address these, researchers from Utrecht University developed an advanced interpreter called Helium which improved the user-friendliness of error messages by limiting the generality of some Haskell features, and in particular removing support for type classes.

Ben Lippmeier designed Disciple[62] as a strict-by-default (lazy by explicit annotation) dialect of Haskell with a type-and-effect system, to address Haskell's difficulties in reasoning about lazy evaluation and in using traditional data structures such as mutable arrays.[63] He argues (p. 20) that "destructive update furnishes the programmer with two important and powerful tools... a set of efficient array-like data structures for managing collections of objects, and ... the ability to broadcast a new value to all parts of a program with minimal burden on the programmer."

Robert Harper, one of the authors of Standard ML, has given his reasons for not using Haskell to teach introductory programming. Among these are the difficulty of reasoning about resource use with non-strict evaluation, that lazy evaluation complicates the definition of data types and inductive reasoning,[64] and the "inferiority" of Haskell's (old) class system compared to ML's module system.[65]

Related languages

Clean is a close, slightly older relative of Haskell. Its biggest deviation from Haskell is in the use of uniqueness types instead of monads for I/O and side-effects.

A series of languages inspired by Haskell, but with different type systems, have been developed, including:

Hume, a strict functional language for embedded systems based on processes as stateless automata over a sort of tuples of one element mailbox channels where the state is kept by feedback into the mailboxes, and a mapping description from outputs to channels as box wiring, with a Haskell-like expression language and syntax.

Conferences and workshops

The Haskell community meets regularly for research and development activities. The main events are:

Notes

↑'Hello world' is meant as the introductory prototype of a read-eval-print_loop. The IO tool putStrLn prints a string, which is the only essential line of this example. The second line of this example is a type definition, which is unnecessary for Haskell, because the compiler infers the type; instead, the second line serves to communicate the programmer's intention to the reader. The first line of the example isn't needed, either, because the start symbol main in this simple example makes the module Main a nicety, which instead would have been a necessity in a multi-module example. Rather, the first two lines are provided for consistency with larger examples.

↑Lattner, Chris (2014-06-03). "Chris Lattner's Homepage". Chris Lattner. Retrieved 2014-06-03. The Swift language is the product of tireless effort from a team of language experts, documentation gurus, compiler optimization ninjas, and an incredibly important internal dogfooding group who provided feedback to help refine and battle-test ideas. Of course, it also greatly benefited from the experiences hard-won by many other languages in the field, drawing ideas from Objective-C, Rust, Haskell, Ruby, Python, C#, CLU, and far too many others to list.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>